This summary table contains detailed information about research studies. Summary tables are a useful way to look at the science behind many breast cancer guidelines and recommendations. However, to get the most out of the tables, it’s important to understand some key concepts. Learn how to read a research table.
Introduction: Birthweight is your weight when you were born.
Women who had a higher birthweight have an increased risk of breast cancer compared to women who had a lower birthweight. This is most clearly seen in premenopausal (before menopause) women.
Learn more about birthweight and breast cancer risk.
Learn about other early life exposures and breast cancer risk.
Learn about the strengths and weaknesses of different types of studies. See how this risk factor compares with other risk factors for breast cancer.
Study selection criteria: Large meta-analyses.
Table note: Relative risk above 1 indicates increased risk. Relative risk below 1 indicates decreased risk.
Study Population(number of participants)
Comparisons of Birthweight
Relative Risk of Breast Cancer in Women with a Higher Birthweight versus Women with a Lower Birthweight, RR (95% CI)
Before or After Menopause
Xue F and Michels KB 
Higher vs. lower birthweight
Park et al. 
6 lbs. 10 oz. - 8 lbs. 13 oz.vs. Less than 6 lbs. 10 oz.
More than 8 lbs. 13 oz.vs. Less than 6 lbs. 10 oz.
Michels KB and Xue F 
* Relative risk for the 4 cohort studies was similar, 1.01 (0.79-1.31).
† Relative risk for the 4 cohort studies was 1.21 (0.80-1.82).
‡ Relative risk for the 5 cohort studies of premenopausal breast cancer was similar, 1.20 (1.07-1.35). Relative risk for the 3 cohort studies of postmenopausal breast cancer was similar, 1.04 (0.91-1.19).
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